1. Field of the Invention
The invention relates to a method for determining the charge state of a motor vehicle battery.
2. Description of the Related Art
A large number of electrical loads are installed in a motor vehicle and they consume electrical power even when the drive unit of the motor vehicle is switched off. These electrical loads include, for example, an anti-theft system or a GPS system of the motor vehicle. The drive unit of the motor vehicle cannot charge the motor vehicle battery when the drive unit is switched off. As a result, the motor vehicle battery of the motor vehicle is discharged when the drive unit is switched off. The discharge of the motor vehicle battery can deactivate the electrical loads of the motor vehicle if a motor vehicle has not been moved for a relatively long period of time. Thus, for example, an anti-theft system of the motor vehicle may no longer be active. In an extreme case, a starter battery of the motor vehicle may even be discharged so that the drive unit of the motor vehicle then can be started only using an external starting aid.
It is already known from practice to connect a battery-charging device to the on-board electrical system of the motor vehicle to counteract such discharging of the motor vehicle battery and deactivation of electrical loads that take up electrical power even when the drive unit is switched off. The battery-charging device can be used to compensate for the power drain of electrical loads and to maintain the motor vehicle battery at a charge state at which the electrical loads which are active even when the drive unit is switched off can be operated reliably.
Battery-charging devices that do not require an electrical mains supply have to be used if an electrical mains supply is not available for operating a battery-charging device. Battery-charging devices that do not require an electrical mains supply may be, for example, a fuel cell battery-charging device.
A battery-charging device that is independent of a mains supply must be used only to charge the motor vehicle battery when it is absolutely necessary due to the charge state of the motor vehicle battery to ensure that the battery-charging device is available for a long time or has a long service life. Accordingly, knowledge of the electrical charge state of the motor vehicle battery of a motor vehicle is of decisive importance for the control of a battery-charging device that is independent of the mains supply.
It is known from practice to determine the electrical charge state of a motor vehicle battery of a motor vehicle on the basis of a voltage of the motor vehicle battery, which is detected by means of measuring technology. However, this only permits relatively inaccurate determination of the electrical charge state of the motor vehicle battery.
Taking this as a basis, the present invention is based on the problem of providing a novel method for determining the charge state of a motor vehicle battery, which method can be used to determine the charge state with a relatively high level of accuracy.